1. Field of the Invention
The present invention relates generally to scanners and scanning methods, and more particularly to a method for compensating for a contaminated calibration target used in calibrating a scanner.
2. Description of the Related Art
An image scanning system must be calibrated to obtain a usable digital image. Typically, such calibration is accomplished by scanning a target image (also known as a calibration strip) with known optical properties, and adjusting analog and digital gains and offsets for pixels of the scanner so that the final digital output correctly matches the target input. The quality of digital scanned images is directly related to the quality of the calibration. In some instances, the quality of the calibration is compromised by contamination of the target image.
Under ideal conditions, the target image, or calibration strip, would be uniform and neutral. However, in reality even tiny imperfections in the uniformity and/or neutrality of the calibration strip can catastrophically effect the calibration of the image scanning system, and thus adversely affect the reproduction quality of subsequent scanned images. These imperfections can be caused by particulate contamination of the calibration strip, or creases in the calibration strip, or any other phenomenon that will alter the optical properties of the calibration strip. For example, since it is virtually impossible to keep the manufacturing environment perfectly clean, it is reasonable to expect that there will be some amount of particulate impurities in the air that may be deposited on the surface of the calibration strip.
When such contamination of the calibration target image occurs, the gains and offsets calculated for the respective pixels of the scanner during the calibration will be incorrect, thereby resulting in displeasing image artifacts in the final digital image of subsequent scanned images.